1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a semiconductor light emitting device reduced in crystal defects and improved in brightness and electrostatic discharge resistance.
2. Description of the Related Art
Recently, a group III-V nitride semiconductor such as GaN layer, or simply a nitride semiconductor has been highlighted as a core material for a light emitting device such as a light emitting diode (LED) or a laser diode (LD) due to superior physical and chemical properties thereof. The group III-V nitride is typically formed of a semiconductor material having a composition expressed by InxAlyGa1-x-yN, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1. This nitride semiconductor light emitting device is applied as a light source for various products such as a keypad light emitting diode of a mobile phone, an electrical sign board, and a lighting device. Particularly, an advance in digital products utilizing the LED or LD has boosted a demand for a nitride semiconductor light emitting device with higher brightness and high reliability. For example, a side view LED used as a backlight of a mobile phone needs to be much brighter and thinner in line with a slimmer trend of the mobile phone.
However, typically, the nitride semiconductor such as GaN layer grown on a heterogeneous substrate such as a sapphire substrate entails many crystal defects due to lattice mismatch with the heterogeneous substrate. These defects seriously undermine reliability of the light emitting device, for example, resistance to electrostatic discharge (ESD). Besides, such defects absorb light to degrade brightness of the light emitting device. In one of many attempts to reduce crystal defects of the nitride semiconductor layer, selective epitaxial growth has been employed. However, the various attempts have entailed a complicated process such as deposition of a SiO2 mask and high costs.
FIG. 1 is a cross-sectional view illustrating a conventional nitride semiconductor light emitting device, particularly, a nitride semiconductor LED. Referring to FIG. 1, the light emitting device 10 includes a sapphire substrate 11, and a buffer layer 13, an n-type GaN layer-based clad layer 14, an active layer 16, and a p-type GaN layer-based clad layer 18 sequentially deposited on the sapphire substrate 11. The n-type GaN layer-based clad layer 14 has a top surface partially exposed by mesa-etching and an n-electrode 24 is formed on the partially exposed top surface. A transparent electrode layer 20 made of e.g., ITO is formed on the p-type GaN layer-based clad layer 18 and a p-electrode 22 is formed on the transparent electrode layer 20. The buffer layer serves to relax lattice mismatch between the sapphire substrate and the n-type GaN layer-based clad layer 14. The buffer layer may be formed of a low-temperature AlN or a low-temperature GaN layer. Japanese Patent Laid-open Publication No. hei 10-135514 discloses an active layer formed of a multiple quantum well structure including an undoped GaN layer barrier layer and an undoped InGaN layer well layer to enhance light emitting efficiency.
However, the buffer layer 13, even when employed, does not sufficiently resolve crystal defects in the light emitting device 10. Still, considerable density defects remain in a nitride semiconductor crystal, notably in the active layer. These defects absorb light and hinder light emission in the active layer. Beside, such defects lower a reverse breakdown voltage (Vr) or a reverse electrostatic discharge (ESD) resistance voltage. These crystal defects accordingly deteriorate brightness and reliability of the light emitting device.